仿射加密

str1 = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
str2 = str1.lower()
a = input("请输入原文")
b = int(input("请输入密钥一"))
c = int(input("请输入密钥二"))
for i in a :
        if i in str2:
            x = str2.find(i)
            y = (b*x + c) % 26
            print(str2[y],end='')
        elif i in str1:
            x = str1.find(i)
            y = (b*x + c) % 26
            print(str1[y], end='')
        elif i in ' ':
            print(' ', end="")
        else:
            print(i, end="")

　　看不懂仿射的解密。。。。。